A. C. Gossard

University of California, Santa Barbara, Santa Barbara, California, United States

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Publications (917)2364.92 Total impact

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    ABSTRACT: We present experimental proof of principle for two-dimensional electrostatic traps for indirect excitons. A confining trap potential for indirect excitons is created by a snowflake-shaped electrode pattern. We demonstrate collection of indirect excitons from all directions to the trap center and control of the trap potential by voltage.
    Optics Letters 02/2015; 40(4):589. · 3.18 Impact Factor
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    ABSTRACT: At low temperatures, indirect excitons formed at the in-plane electron-hole interface in a coupled quantum well structure undergo a spontaneous transition into a spatially modulated state. We report on the control of the instability wavelength, measurement of the dynamics of the exciton emission pattern, and observation of the fluctuation and commensurability effect of the exciton density wave. We found that fluctuations are strongly suppressed when the instability wavelength is commensurate with defect separation along the exciton density wave. The commensurability effect is also found in numerical simulations within the model describing the exciton density wave in terms of an instability due to stimulated processes.
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    ABSTRACT: Collective vibrations of proteins, rotations of small molecules, excitations of high-temperature superconductors, and electronic transitions in semiconductor nanostructures occur with characteristic frequencies between 1 and 10 THz. Applications to medicine, communications, security and other fields are emerging. However, mapping the coldest parts of the universe has been the largest driver for developing THz detectors. The result is a family of exquisitely-sensitive detectors requiring sub-4K temperatures. For earthbound THz science and technology, sensitivity remains important but many applications require high speed and operating temperatures. Room-temperature Schottky diodes enable some of these applications. Here we demonstrate a new type of detector in which THz radiation excites a collective oscillation of ~25,000 electrons between two gates in a microscopic four terminal transistor. The energy dissipates into other modes of the electron gas, warming it and changing the source-drain resistance. The detector shows amplifier-limited rise times near 1 ns and has detected THz laser radiation at temperatures up to 120K. The frequency of the collective oscillation tunes with small gate voltages. The first-generation tunable antenna-coupled intersubband Terahertz (TACIT) detectors tune between 1.5 and 2 THz with voltages <2V.
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    ABSTRACT: Advances in thin film growth technology have enabled the selective engineering of material properties to improve the thermoelectric figure of merit and thus the efficiency of energy conversion devices. Precise characterization at the operational temperature of novel thermoelectric materials is crucial to evaluate their performance and optimize their behavior. However, measurements on thin film devices are subject to complications from the growth substrate, non-ideal contacts, and other thermal and electrical parasitic effects. In this manuscript, we determine the cross-plane thermoelectric material properties in a single measurement of a 25 μm InGaAs thin film with embedded ErAs (0.2%) nanoparticles using the bipolar transient Harman method in conjunction with thermoreflectance thermal imaging at temperatures up to 550 K. This approach eliminates discrepancies and potential device degradation from the multiple measurements necessary to obtain individual material parameters. In addition, we present a strategy for optimizing device geometry to mitigate the effect of both electrical and thermal parasitics during the measurement. Finite element method simulations are utilized to analyze non-uniform current and temperature distributions over the device area as well as the three dimensional current path for accurate extraction of material properties from the thermal images. Results are compared with independent in-plane and 3ω measurements of thermoelectric material properties for the same material composition and are found to match reasonably well; the obtained figure of merit matches within 15% at room and elevated temperatures.
    Journal of Applied Physics 07/2014; 116(3):034501-034501-9. · 2.19 Impact Factor
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    ABSTRACT: Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.
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    ABSTRACT: We present new high-resolution measurements of transient time-domain photoconductivity in ErAs:InGaAs superlattice nanocomposites intended for THz photoconductive switches and photomixers using a pure optical pump-probe method. We developed a model, using separate photocarrier trapping, recombination, and thermal reactivation processes, which very accurately fits the measurements. The measured material structures all exhibit a slow secondary decay process, which is attributed to thermal reactivation of the trapped carriers, either into the conduction band, or into high-energy defect states. We examined the influence of superlattice structure, dopants, DC bias, and temperature. Analysis shows that all of the THz energy produced by the photocarrier trapping and decay processes are at frequencies less than 1 THz, while the reactivation process only serves to create a large portion of the bias power dissipated. Energy higher than 1 THz must be created by a fast generation process or band-filling saturation. This allows pulsed THz generation even from a long-lifetime material. Pure optical pump-probe measurements are necessary to expose slow material processes, and eliminate the influence of electrical terminals and THz antennas. These measurements and modeling of THz photoconductive devices are necessary in order to optimize the output spectrum and power.
    Journal of Applied Physics 07/2014; 116(1):013703-013703-8. · 2.19 Impact Factor
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    ABSTRACT: We report on self-assembled ErSb nanowires in a GaSb matrix that show a strong polarization-sensitive THz response. The nanowires behave like a polarizer. Their orientation and shape can be engineered by the growth conditions.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: Transport, relaxation, and correlation effects are observed for indirect excitons in high magnetic fields.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: We experimentally demonstrate an order of magnitude higher radiated power from a 1550 nm photomixer with plasmonic contact electrodes in comparison with an analogous photomixer without plasmonic contact electrodes in the 0.25-2.5 THz frequency range.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: We report the observation of spin currents and spin polarization textures in opti- cally generated indirect excitons. The textures are observed in linear and circular polarizations and are controlled by magnetic fields.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: While the growth of III-As and III-P semiconductors is well-established, and their transport properties well-understood, the performance of high-frequency and VLSI electron devices can still be substantially improved. Here we review design principles, experimental efforts, and intermediate results, in the development of nm and THz electron devices, including nm InAs/InGaAs planar MOSFETs and finFETs for VLSI, InGaAs/InP DHBTs for 0.1-1 THz wireless communications and imaging, and ~5nm InAs/InGaAs Schottky diodes for mid-IR mixing.
    2014 72nd Annual Device Research Conference (DRC); 06/2014
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    ABSTRACT: We present Silver-epoxy filters combining excellent microwave attenuation with efficient wire thermalization, suitable for low temperature quantum transport experiments. Upon minimizing parasitic capacitances, the attenuation reaches >100 dB above ~150 MHz and - when capacitors are added - already above ~30 MHz. We measure the device electron temperature with a GaAs quantum dot and demonstrate excellent filter performance. Upon improving the sample holder and adding a second filtering stage, we obtain electron temperatures as low as 7.5 +/- 0.2 mK in metallic Coulomb blockade thermometers.
    Applied Physics Letters 03/2014; 104(21). · 3.52 Impact Factor
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    ABSTRACT: Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.
    Applied Physics Letters 03/2014; 104(9):091101. · 3.52 Impact Factor
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    ABSTRACT: Multielectron spin qubits are demonstrated, and performance examined by comparing coherent exchange oscillations in coupled single-electron and multielectron quantum dots, measured in the same device. Fast (>1 GHz) exchange oscillations with a quality factor Q∼15 are found for the multielectron case, compared to Q∼2 for the single-electron case, the latter consistent with experiments in the literature. A model of dephasing that includes voltage and hyperfine noise is developed that is in good agreement with both single- and multielectron data, though in both cases additional exchange-independent dephasing is needed to obtain quantitative agreement across a broad parameter range.
    Physical Review Letters 01/2014; 112(2):026801. · 7.73 Impact Factor
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    ABSTRACT: We present measurements of the electron temperature using gate defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.
    Journal of Low Temperature Physics 01/2014; 175(5-6). · 1.04 Impact Factor
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    ABSTRACT: We report on Terahertz (THz) detectors based on III-V high-electron-mobility field-effect transistors (FET). The detection results from a rectification process that is still highly efficient far above frequencies where the transistor provides gain. Several detector layouts have been optimized for specific applications at room temperature: we show a broadband detector layout, where the rectifying FET is coupled to a broadband logarithmic-periodic antenna. Another layout is optimized for mixing of two orthogonal THz beams at 370 GHz or, alternatively, 570 GHz. A third version uses a large array of FETs with very low access resistance allowing for detection of very short high-power THz pulses. We reached a time resolution of 20 ps.
    Proceedings of SPIE - The International Society for Optical Engineering 10/2013; · 0.20 Impact Factor
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    ABSTRACT: Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements and non-orthogonal control axes.
    Nature Nanotechnology 09/2013; · 31.17 Impact Factor
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    ABSTRACT: We report Te doping of bulk In0.53Ga0.47As up to 2.6×1019 cm−3 without saturation effects, and structural characterization and contact resistances between metal and epitaxial regrowth for four structures: Si doped and Si and Te co-doped n+ InAs regrowth on a 10 nm In0.53Ga0.47As channel, Si doped and Si and Te co-doped n+ In0.53Ga0.47As regrowth on a 7 nm In0.53Ga0.47As channel. We observe that the contact resistance for the Si doped and Si and Te co-doped n+ InAs regrowth on the 10 nm In0.53Ga0.47As channel is 9.9 Ω μm (3.9 Ω μm2) and 6.6 Ω μm (2.3 Ω μm2), and the contact resistance for the Si doped and Si and Te co-doped n+ In0.53Ga0.47As regrowth on the 7 nm In0.53Ga0.47As channel is 8.5 Ω μm (2.3 Ω μm2) and 6.8 Ω μm (1.9 Ω μm2). The improvement in contact resistance comes from improvements in electron mobility consistent with Te improving material quality as a surfactant.
    Journal of Crystal Growth 09/2013; 378:92-95. · 1.69 Impact Factor
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    ABSTRACT: Given low interface trap densities and low access resistances, InGaAs MOSFETs can provide greater on-state current than silicon MOSFETs at the same effective oxide thickness (EOT), and are thus strong candidates for use in VLSI.1 Transconductance as high as 2.1 mS/μm (Vds=0.5 V) with 115 mV/decade (Vds=0.5 V) subthreshold swing has been reported2 in planar III-V MOSFETs using a gate recess etch through the N+ InGaAs contact layer. It remains to be established whether the necessary etch depth control can be obtained at VLSI integration scales and 10-20 nm gate lengths. Using self-aligned regrowth of the N+ source and drain, III-V MOSFETs can be fabricated without requiring this gate recess etch;3 1.9 mS/μm (Vd =1 V) with 116 mV/decade (Vds=0.05 V) subthreshold swing was reported in a 55 nm Lg InGaAs MOSFET with MOCVD source-drain regrowth.4 Note that no post-regrowth etching of the channel surface is reported in (4). We have recently found5 that InGaAs MOSFETs using MBE source-drain regrowth, subthreshold swing and transconductance are substantially improved by removing a 5 nm N+ InGaAs channel cap post-regrowth and immediately prior to gate dielectric deposition, suggesting damage to the channel surface during regrowth. Here, we report similar findings for MOCVD regrowth. We fabricated 65 nm Lg In0.53Ga0.47As surface-channel MOSFETs in a gate-last process with self-aligned raised InGaAs S/D access regions formed by MOCVD regrowth. Removal of ~ 2.4 nm of the channel surface by digital etching improved the transconductance from 1.1 to 1.58 mS/μm (65 nm Lg Vd=0.5 V), and reduced the subthreshold swing from 326 to 110 mV/dec (1 μm Lg, Vds=0.05 V). These results suggest that substantial surface damage arises, and must be addressed, in MOCVD reg- owth III-V MOSFET processes.
    2013 71st Annual Device Research Conference (DRC); 06/2013

Publication Stats

30k Citations
2,364.92 Total Impact Points


  • 1989–2015
    • University of California, Santa Barbara
      • • Department of Electrical and Computer Engineering
      • • Department of Physics
      Santa Barbara, California, United States
  • 2013
    • GlobalFoundries Inc.
      Schenectady, New York, United States
  • 1990–2013
    • Harvard University
      • Department of Physics
      Cambridge, MA, United States
  • 2006–2012
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1984–2010
    • Princeton University
      • • Department of Physics
      • • Department of Electrical Engineering
      Princeton, NJ, United States
  • 2006–2009
    • ETH Zurich
      • Laboratory for Solid State Physics
      Zürich, ZH, Singapore
  • 2008
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      • Institute of Optics, Information and Photonics
      Erlangen, Bavaria, Germany
    • Nuremberg University of Music
      Nuremberg, Bavaria, Germany
  • 2000–2008
    • Macalester College
      • Department of Physics and Astronomy
      Saint Paul, MN, United States
  • 2007
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, MA, United States
  • 2002–2007
    • Universitätsklinikum Erlangen
      Erlangen, Bavaria, Germany
  • 2004
    • University of Florida
      • Department of Physics
      Gainesville, FL, United States
  • 2000–2002
    • University of California, Berkeley
      • Department of Physics
      Berkeley, MO, United States
  • 2001
    • University of Chicago
      • James Franck Institute
      Chicago, Illinois, United States
  • 1999
    • Ludwig-Maximilians-University of Munich
      • Center for Nanoscience (CeNS)
      München, Bavaria, Germany
    • Vienna University of Technology
      • Institute of Solid State Electronics
      Wien, Vienna, Austria
  • 1994–1999
    • CSU Mentor
      Long Beach, California, United States
  • 1998
    • University of Vienna
      Wien, Vienna, Austria
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 1996
    • KTH Royal Institute of Technology
      • Department of Physics
      Tukholma, Stockholm, Sweden
  • 1992–1996
    • Linköping University
      • Department of Physics, Chemistry and Biology (IFM)
      Linköping, Östergötland, Sweden
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
  • 1985–1989
    • AT&T Labs
      Austin, Texas, United States
  • 1982–1989
    • University of Innsbruck
      • Institut für Experimentalphysik
      Innsbruck, Tyrol, Austria
    • The University of Arizona
      • College of Optical Sciences
      Tucson, Arizona, United States
    • National Institute of Applied Science
      Strasburg, Alsace, France
  • 1983
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany